Optical Swapping of Digitally-Encoded Optical Labels

ABSTRACT

A method and a device are provided for swapping optical labels in an optical communication network. Optical information, including payload data and label data digitally encoded into the optical information, is received. At least one group of bits within the optical information is selectively inverted to rewrite the label data with new label data without changing the payload data. Each of the at least one group of inverted bits includes at least two bits and all bits of each of the at least one group of inverted bits are contiguous bits.

PRIORITY INFORMATION

This application is a continuation of U.S. patent application Ser. No.11/283,977, filed Nov. 21, 2005, the contents of which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical networking, and moreparticularly, to a method and apparatus for optical swapping ofdigitally-encoded label information within an optical stream or packet.

2. Introduction

Packet networks transport data from a source location to a destinationlocation by organizing the data into self-contained units calledpackets. Each packet carries its routing information as it passesthrough a series of routing nodes on its way to the destinationlocation. Each routing node reads the routing information associatedwith the packet and uses that information to decide the correct path touse to forward the packet. In traditional IP (Internet Protocol)networks, the routing information is made up of individual addresses ofsource and destination nodes. In more advanced MPLS (Multiprotocol LabelSwitching) networks, packets are assigned additional labels that groupthem according to their intermediate or final destinations. This labelassignment promotes efficient scaling and quality-of-service assignmentin the MPLS networks. In both IP and MPLS networks with opticaltransport between routing nodes, all packets are converted from opticalto electrical form as they enter the routing node and then convertedback to optical form as they leave the routing node. Minimization ofsuch Optical-Electronic-Optical (OEO) conversions is a central principleof cost reduction in advanced optical networks. Removing OEO conversionsand electronic switching also helps with scaling the routers to massivecapacity, since these electronic functions contribute to the buildup ofcost, failure rate, and power dissipation in the router nodes.

As a result, Optical Label Switching (OLS) networks are under intensivestudy as a means of combining the flexibility and statisticalmultiplexing of electronic IP packet networks with thecost-effectiveness and massive scalability of optical data transport.OLS networks include an optical label (OL) with each packet of payloaddata, and the OL is read at each routing node to determine the properswitch settings for packet forwarding. OLs may be in-band, sent asheaders occupying the first bytes of every packet, but that approachrequires expensive photoreceivers capable of operating at full datarate. Thus, OLs are usually sent in a separate out-of-band channel. Forflexibility and scaling, it is desirable to be able to change a value ofan optical label as it passes through a routing node. This is known aslabel swapping.

The value of OLS networks is greatly enhanced when they can carrymultiple optical packets simultaneously. Such a capability can beimplemented through the use of wavelength division multiplexing (WDM),in which each packet is assigned to a specific wavelength of light.Using WDM, multiple simultaneous packets are combined at the source withan arrangement of optical wavelength filters called a wavelengthmultiplexer (MUX), and re-separated before detection at the destinationwith a reciprocal arrangement of optical filters called a wavelengthdemultiplexer (DMUX). WDM presents OLS networks with an additionalchallenge of reading multiple OLs that are simultaneously present at anygiven point in the network. For in-band labels, the straightforwardsolution is to follow a DMUX with a parallel array of label receivers,but this becomes expensive as the wavelength count becomes large.Alternatively, one might place a tunable wavelength selection filterbefore a shared label receiver, but this would reduce packet throughputand demand extremely complex network synchronization.

Various of out-of-band OL technologies have been proposed for WDM OLSnetworks. Some use dedicated label wavelengths for each packetwavelength, reducing spectral efficiency of the networks. Others rely onorthogonal modulation formats, such as optical phase shift keying (PSK)for labels in combination with amplitude shift keying (ASK) forpayloads. Although this approach can reduce the number of full-rate OEOconversions and enable optical label switching, it often has flaws suchas complex modulation formats, crosstalk caused by optical impairments,or high cost.

Another key element of optical packet networks is the all-opticalregenerator. As packets pass through optical transmission lines that maybe hundreds or thousands of km long, they accumulate impairments thatdegrade the quality of the pulses that represent data bits. If notcorrected, these impairments will cause bit errors and corrupt thepackets. Regenerators are non-linear signal processing subsystems thatrestore the correct amplitude, pulse shape, and timing to each bit ofthe packet. To minimize OEO conversions, all-optical regenerators havebeen developed, and used to demonstrate error-free data transmission upto 1 million km. All-optical regenerators can be classed asnon-inverting or inverting, depending on whether or not 1s are exchangedwith 0s in the regeneration process. Because the inversion affects allbits in exactly the same way, the original data can be easily recoveredby an additional inversion process at the destination.

Optical labels can also be useful in circuit-switched networks,especially those capable of dynamically re-routing signals on awavelength-by-wavelength basis.

Thus, there is a need for a practical method of encoding optical labelscarrying routing information or other information in optical packetnetworks and also for a method of optical label swapping applicable toboth packet-switched and circuit-switched optical networks.

SUMMARY OF THE INVENTION

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present inventionwill become more fully apparent from the following description andappended claims, or may be learned by the practice of the invention asset forth herein.

In a first aspect of the invention, a method for swapping optical labelsin an optical communication network is provided. Optical information,including payload data and label data digitally encoded into the opticalinformation, is received. At least one group of bits within the opticalinformation is selectively inverted to rewrite the label data with newlabel data without changing the payload data. Each of the at least onegroup of inverted bits includes at least two bits and all bits of eachof the at least one group of inverted bits are contiguous bits.

In a second aspect of the invention, a device for swapping opticallabels in an optical network is provided. The device includes a labelprocessing component and a selective inversion device. The labelprocessing component is arranged to receive first optical label data andsecond optical label data to produce a difference signal representing adifference of bit settings between the first optical label data and thesecond optical label data. The selective inversion device is arranged toreceive an optical signal and the difference signal and to selectivelyinvert groups of at least two contiguous bits in the optical signal suchthat the first optical label data encoded within the optical signal isrewritten to be equal to the second optical label data without changingpayload data represented by the encoded payload data in the opticalsignal.

In a third aspect of the invention, a device for receiving an opticalsignal in an optical network is provided. The device includes firstlabel data and second label data to produce a difference signalrepresenting a difference of bit settings between the first label dataand the second label data, and means for receiving an optical signal andthe difference signal and for selectively inverting groups of at leasttwo contiguous bits in the optical signal such that the received firstlabel data is rewritten to be equal to the second label data withoutchanging payload data represented by the encoded payload data in theoptical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 illustrates an exemplary optical network in which embodimentsconsistent with principles of the invention may be implemented;

FIG. 2A illustrates an exemplary apparatus for encoding and transmittingoptical data consistent with principles of the invention;

FIG. 2B shows aspects of exemplary encoding of coded payload data with achip stream;

FIG. 3 is a flowchart that describes an exemplary encoding processconsistent with the principles of the invention;

FIG. 4 illustrates an exemplary label receiver, consistent with theprinciples of the invention, for detecting a composite optical signaland recovering label data from the composite optical signal;

FIG. 5 is a functional block diagram that illustrates an optical networkrouting node incorporating an exemplary label swapping device;

FIG. 6 is a functional block diagram that illustrates an exemplary labelswapping device for swapping optical labels in a single wavelengthoptical network; and

FIG. 7 is a functional block diagram that illustrates an exemplary labelswapping device for swapping optical labels in a multiple wavelengthoptical network.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments of the invention are discussed in detail below.While specific implementations are discussed, it should be understoodthat this is done for illustration purposes only. A person skilled inthe relevant art will recognize that other components and configurationsmay be used without parting from the spirit and scope of the invention.

Introduction

U.S. patent application Ser. No. 11/101,778, filed on Apr. 8, 2005 andincorporated by reference herein in its entirety, introduces a newdigital encoding process optimized for path tracing in circuit-switchedoptical networks with minimal OEO conversion. Using complementaryconstant weight codes, this encoding process may embed an auxiliarychannel with management information into high-speed payload data in sucha way that the management information can be extracted by low costphotoreceivers without a need to process, or even detect, the high-speedpayload data. For WDM networks, a layer of Code Division Multiple Access(CDMA) coding may be added to enable a single auxiliary receiver tocapture and decode individual label data from multiple wavelengthssimultaneously. For more advanced reconfigurable networks, it may bedesirable to rewrite optical labels without OEO conversion at networkelements such as wavelength converters or all-optical regenerators.

U.S. patent application Ser. No. 11/283,978, entitled “DIGITAL ENCODINGOF LABELS FOR OPTICAL PACKET NETWORKS”, Inventors: Mark D. Feuer andVinay Vaishampayan, filed on the same date as the present applicationand herein incorporated by reference in its entirety, discloses adigital encoding and decoding process, similar to that described in U.S.patent application Ser. No. 11/101,778, but for use with optical packetnetworks.

Although aspects of the following describe implementations in opticalpacket switching networks, embodiments of the present invention may beused with either an optical circuit-switching network or an opticalpacket-switching network.

Exemplary Optical Network

FIG. 1 illustrates an exemplary optical network 100 which may includeimplementations consistent with the principles of the invention. Foreach network path, one node may be designated as a source node 101, oneor more nodes may be designated as intermediate nodes, 103, 104 and 105,and one or more nodes may be designated as destination nodes 102. Eachnode should have a connection to at least one other node and thereshould be a path or route between any source node 101 and destinationnode 102.

Exemplary Encoding Implementations

FIG. 2A illustrates an exemplary apparatus that may be included insource nodes 101 of optical network 100. The apparatus may include adigital encoder 200 and a laser transmitter 202. In some implementationsconsistent with the principles of the invention, the apparatus mayinclude a wave division multiplexer (WDM) 204.

Digital encoder 200 may include coder 206, which may encode each groupof K bits of raw payload data into a block of N bits of coded payloaddata, where K<N. Digital encoder 200 may employ a complementaryconstant-weight code (CCWC) with 2^(K+1) codewords. Half of thecodewords may have a constant weight of W, where W<N/2, and a secondhalf of the codewords may have a constant weight of N−W, obtained byinverting or complementing each codeword having a constant weight of W.Each group of K bits of the raw payload data may always be encoded bycoder 206 into a block of N bits of coded payload data by using acodeword of constant weight W. Thus, the weight of the blocks of codedpayload data may be uniform, and the average power may be constant.

A label message may include routing information for an optical packet orother useful information. In some implementations consistent with theprinciples of the invention, each label message may be spread by asignature sequence such as, for example, a CDMA signature sequence, bypassing the label message and the signature sequence through anexclusive OR (XOR) gate, to produce binary chip data. The rate of theproduced binary chip data is the faster rate of the label data and thesignature sequence. For example, the label data may have a rate of about50 kilobits per second and the signature sequence may have a rate ofabout 10 Megabits per second. Thus, the resulting binary chip data mayhave a rate of about 10 Megabits per second. Further, because of thedifferences in data rates between the label message and the signaturesequence, each bit of the label message, when combined with thesignature sequence may be XOR'ed with many bits of the signaturesequence, thereby producing many bits of binary chip data. The number ofchips of binary chip data that result from combining one label messagebit with the signature sequence is C.

Next, the blocks of coded payload data may be XOR'ed with the binarychip data to produce coded composite data blocks. This XOR process mayreplace some of the blocks with their complements, producing codedcomposite data whose average power shows a binary modulation accordingto the chip stream, with effective modulation index (1-2W/N). For asingle label message frame, the composite data may have a length of Cchips of B data blocks each, where each block comprises N bits ofcomposite data. In one specific implementation, N may be 1,024, B may be1, and C may be 200, thereby making the label frame size in thisimplementation 204,800 bits.

The CDMA layer may provide a mechanism for many packets on differentwavelengths in a WDM network to share a single all-wavelength labelreceiver. The CDMA-based spectrum spreading may also contribute to alonger averaging interval that enhances immunity to optical amplifiernoise. The fully-coded composite data, combining both payload data andlabel frames in a single binary sequence, may be fed to a standardoptical transmitter such as, for example, laser transmitter 202, toproduce an on-off-keyed (OOK) optical signal that can be combined withsimilarly coded optical signals at other wavelengths by a wave divisionmultiplexer (WDM) 204 for transmission through the optical packetnetwork.

Although the above-described implementation may use constant-weightcodes, in other implementations other types of complementary codes maybe used. Further, the CDMA layer is optional and may be replaced by adifferent multiple access strategy, such as Orthogonal FrequencyDivision Multiple Access, or omitted entirely in non-WDM networks. Inalternative implementations consistent with the principles of theinvention, two complementary code blocks may represent each pattern ofpayload bits, and the label frame may be encoded into the weight ofthese two alternative code blocks. Also, in single-wavelengthimplementations, the coded payload data may be XOR'd with the bits ofthe label data instead of with the chip stream. However, because thepayload data rate is much higher than the label data rate, each bit oflabel data may be XOR'ed with many bits of coded payload data.

FIG. 2B illustrates encoding of a portion of chip data onto a portion ofcoded payload data to produce coded composite data 250 in oneimplementation consistent with the principles of the invention. Codedcomposite data 250 is shown as having N bits per block, B blocks perchip, and C chips per frame. Chip data portion 252 is shown as chips010001, which are to be encoded onto the portion of the coded payloaddata to produce coded composite data 250. In the example of FIG. 2B,each chip of chip data 252 may be encoded into N*B bits of codedcomposite data 250. In one implementation, each 0 chip of chip data 252is encoded over B blocks of N bits of coded payload data producing N*Bbits of coded composite data 250 in which each block of N bits has anaverage power of W/N; and each 1 chip of chip data 252 is encoded over Bblocks of N bits of coded payload data producing N*B bits of codedcomposite data 250 in which each block of N bits has an average power of(N−W)/N. Thus, chip data 252 may be recovered from coded composite data250 based on the average power of B blocks of N bits each.

Although the above example describes each 0 chip of chip data 252 beingencoded into coded composite data 250 as a group of blocks having anaverage power of W/N, and describes each 1 chip of chip data 252 beingencoded into coded composite data 250 as a group of blocks having anaverage power of (N−W)/N, each 0 chip of chip data 252 may be encodedinto a group of blocks having an average power of (N−W)/N, and each 1chip of chip data 252 may be encoded into a group of blocks having anaverage power of W/N.

FIG. 3 is a flowchart that helps to illustrate an exemplary process thatmay be used to encode label data into optical signals in animplementation consistent with the principles of the invention. Groupsof K payload bits may be encoded, via coder 206, into N bit blocks ofcoded payload data using codewords of a CCWC code having a constantweight of W (act 300). In parallel, information from a label message maybe XOR'ed with a signature sequence such as, for example, a CDMAsignature sequence, to produce chip data having a rate that may be aninteger multiple of the label rate (act 302). Next, each group of Bblocks of N bits of the coded payload data may be XOR'ed with one chipof the chip data to produce a portion of coded composite data (act 304).Thus, a 1 bit of the chip data may invert N*B bits of coded payloaddata, having a constant weight of W*B, to produce N*B bits of the codedcomposite data having a constant weight of (N−W)*B. An opticaltransmitter, such as laser transmitter 202, may produce an opticalsignal of a particular wavelength, corresponding to the coded compositedata (act 306). One or more produced optical signals of otherwavelengths may be optically combined with the coded composite data viaWDM 204 to be transmitted as optical signals via an optical network (act308).

Exemplary Decoding Implementations

FIG. 4 illustrates an exemplary label receiver 400 for detecting anddecoding label frames encoded onto optical packets in an optical packetnetwork by an apparatus such as, for example, the exemplary apparatus ofFIG. 2A. Label receiver 400 may be included in intermediate nodes suchas, for example, routing nodes of an optical network. Label receiver 400may include a photodiode (PD) 402 such as, for example, a slow PD, achip rate integrator 404, a sampler 405 and a decoder 406 such as, forexample, a CDMA decoder.

Multiple wavelengths may impinge on PD 402, which is capable ofresponding at the chip rate. That is, PD 402 may have a maximumfrequency of operation that is less than one-half of the rate ofreceived composite data. The resulting electrical signal is made up of asuperposition of chip data from the various wavelengths. Chip rateintegrator 404 and sampler 405 may perform sampling such as, forexample, integrate-and-dump sampling, clocked to the (synchronous) chipdata to assure complete rejection of the payload data. Decoder 406 mayoperate on analog chip samples from sampler 405 to recover individualoptical labels.

Due to simplicity of the label receiver design, the digital labelencoding method can achieve significant cost benefits over a system inwhich in-band labels are incorporated into conventional packet headers.A conventional packet scheme requires multiple 10 Gb/s receivers, onefor each wavelength. In contrast, the new label encoding method may usea single photodiode and accompanying circuitry that is significantlyless expensive, operating at ˜10 Mb/s. Also, the low-speed chip data canbe received at a lower optical power, allowing operation from a low-lossoptical tap and eliminating a need for additional optical amplifiers.Finally, a CDMA layer may allow simultaneous reception of labelsattached to multiple wavelengths. Digital label encoding also offers anenhanced degree of data privacy. Depending on the throughput demandedfor label data, it is possible to operate the apparatus at a power levelso low that photon shot noise would obscure the payload information.

Exemplary Label Swapping Device

FIG. 5 illustrates an optical routing node incorporating an exemplarylabel swapping device 504 for use in an optical network. The opticalrouting node may include an optical power tap 502, label receiver 400, alabel swapping device 504 and an optical switch 506.

Optical power tap 502 may extract a fraction of the optical signal fromthe network. The optical signal may include composite data havingencoded payload data and encoded label data such that the encoded labeldata may be superposed with respect to the encoded payload data asdescribed previously. The tapped fraction of the optical signal may bepassed to label receiver 400 for recovery of the label data. Therecovered label data may be passed to label swapping device 504 and tooptical switch 506. The untapped part of the optical signal may bepassed to label swapping device 504 for selectively inverting groups ofcontiguous bits. The label data included in the encoded label data mayinclude routing information or other useful information. Label swappingdevice 504 may output an optical data stream having swapped or rewrittenlabel data. Optical switch 506 may receive the output optical datastream, may change the switch settings based on the label data, and mayforward the optical signal toward its destination.

The optical signal in FIG. 5 may consist of a single wavelength ormultiple wavelengths, and if it consists of multiple wavelengths, eachwavelength may have its own payload data and its own label data. If theoptical signal is a multi-wavelength signal, both the label swappingdevice 504 and the optical switch 506 may operate on each wavelengthindependently, to relabel each wavelength and forward it towards itsdestination. In one implementation of a single wavelength node, labelreceiver 400 may not include CDMA decoder 406. Instead, a single labelmay be output from sampler 405.

FIG. 6 illustrates a detailed view of an implementation of labelswapping device 504 for a single-wavelength optical network. Labelswapping device 504 may include selective inversion device 604 and XORdevice 602.

The single recovered label may be provided to a label processor, whichin this implementation may be XOR device 602. XOR device 602 may also beprovided with a second label, which may be a desired new label for theoptical packet. Label processor, or XOR device 602, may output adifference signal, which may indicate the bit differences between thefirst label and the second label. The one bits of the difference signalmay indicate the groups of bits in the optical signal that are to beselectively inverted.

Selective inversion device 604 may include an all-optical regenerator oran interferometric device. The interferometric device may have at leastone optical phase shift adjustment mechanism. In one implementationconsistent with the principles of the invention, selective inversiondevice 604 may selectively invert one or more label frames or groups ofN×B×C bits of encoded payload data based on the bit pattern of thedifference signal, to effectively invert selected bits or frames of theencoded label data superposed with respect to the encoded payload data.Selective inversion device 604 may then output the optical signal withthe new label data.

FIG. 6 may be modified to be a label swapping device operating in amulti-wavelength network by making selective inversion device 604capable of performing its function on multiple wavelengthssimultaneously, and by generalizing the first label data, second labeldata, and XOR 602 to operate with multiple parallel channels, one foreach wavelength.

FIG. 7 illustrates an exemplary label swapping device 504′ which may beused in implementations consistent with the principles of the inventionthat receive optical signals including multiple wavelengths of light.Label swapping device 504′ may include label processor 708, wavelengthdemultiplexer (WDM) 702, a group of single wavelength inversion devices604, and wavelength multiplexer (MUX) 706.

Label swapping device 504′ may receive an optical signal having multiplewavelengths. It may also receive recovered label data for eachwavelength, as previously described with respect to FIG. 4. In oneimplementation consistent with the principles of the invention, labelprocessor 708 may include a group of XOR devices, one for eachwavelength. Label receiver 400 (not shown in FIG. 7) may provide themultiple labels to label processor 708. Label processor 708 may also beprovided with new label data. Label processor 708 may XOR each of thereceived labels with a respective new label, thereby producing adifference signal for each wavelength. The difference signal may be abit pattern indicating which groups of bits in the optical wavelengthsignal are to be inverted such that the resulting label is equal to thenew label.

DMUX 702 may receive the multiple wavelength optical signal and maydemultiplex the signal into a respective signal for each wavelength. Therespective signal may be input to a respective one of the selectiveinversion devices 604, which may also receive a respective one of thedifference signals. Each selective inversion device 604 may selectivelyinvert groups of bits corresponding to a label frame or frames,according to the respective difference signal, to swap or rewrite therespective label to a desired new label value. Each selective inversiondevice 604 may output a single wavelength optical signal. Each singlewavelength optical signal may then be multiplexed by MUX 706 to producea multiple wavelength optical signal with new labels.

CONCLUSION

Although the above description may contain specific details, they shouldnot be construed as limiting the claims in any way. Other configurationsof the described embodiments of the invention are part of the scope ofthis invention. For example, although the above embodiments aredescribed as producing an OOK optical signal, other keying methods maybe used such as, for example, amplitude shift keying (ASK) or othermethods. Further, implementations consistent with the principles of theinvention may have more or fewer acts than as described, or mayimplement acts in a different order than as shown. Accordingly, theappended claims and their legal equivalents should only define theinvention, rather than any specific examples given.

1. A method for swapping optical labels in an optical communicationnetwork, the method comprising: receiving optical information includingpayload data and label data digitally encoded into the received opticalinformation; selectively inverting at least one group of bits within theoptical information to rewrite the label data with new label datawithout changing the payload data, wherein: each of the at least onegroup of inverted bits comprises at least two bits, and all bits of eachof the at least one group of inverted bits are contiguous bits.